It is occasionally necessary to at least limitedly move a tool such as a milling or planning head along its rotation axis relative to the support of the machine having the tool. To this end the spindle or shaft carrying the tool and defining the rotation axis therefor is normally mounted in bearings carried on a mount that itself can be displaced relative to the machine support. A threaded spindle engaging between the mount and the support and extending parallel to the shaft axis is rotatable about its own axis for the necessary axial adjustment.
The disadvantage of this system is that it is relatively complex and adds considerably to the cost of the machine. Furthermore unless the tolerances are extremely good in the positioning mechanism, some play will be present and vibration will result.
On simpler and normally less expensive tools it is standard practice simply to mount the tool on the shaft with interposition of a stack of spacer washers between the tool and holding nut on one side and a shoulder on the other. By removing spacers from one side and placing them on the other side of the tool it is possible to position this tool axially along the shaft within the limited range normally necessary for such machines. This is a relatively onerous operation and cannot be used to provide a stepless positioning of the tool on the shaft.